Home Brewing. Boiling and Cooling

14 June 2011

The boil is one of the most important stages in the brewing process, although it is the simplest to perform. The boil destroys any bacteria that may be present in the wort, destroys the enzymes to prevent further saccharification, drives off undesirable compounds in the wort, extracts the bit-tering substances from the hops, and caramelises some of the sugars which provide colour and flavour. The boil also causes proteins to coagulate and settle out, thereby improving the clarity and stability of the finished beer.

A typical boiling period is from one to two hours. Generally, the longer the boil the better the stability of the finished beer. The old Burton brewers used to boil for three hours, but few modern commercial brewers boil for that length of time for reasons of economy; one and a half hours is more usual. It is during the boil that the hops and any sugars or syrups called for in the recipe are added to the wort. The hops are usually added when the wort begins to boil, but sometimes about 25 per cent of the hops are held back and added during the last fifteen minutes. This is done because some of the flavour and aroma giving substances derived from the hops are volatile and are driven off during the boil, although the bitterness and preservative properties of the hops still remain. This last 25 per cent of the hops are known as late hops and should be of the very best quality.

Sugars are usually added to the copper about half way through the boil, but the exact point at which they are added is quite unimportant.

A substance which is often added during the boil is copper finings, which serve to assist in the clarification of the wort during and after the boil. The most commonly used copper fining agent is Irish moss, sometimes called carragheen, but it is in fact a dried seaweed or substances extracted from it. Colloids extracted from the Irish Moss during the boil combine with proteins and other solid particles in the wort, assisting them to settle out when the boil is completed. The colloids extracted from the moss are negatively charged, which attract positively charged particles (proteins). These then combine and settle out. Some residual extract from the moss will carry across to the finished beer and act as auxiliary finings.

Driving off undesirable substances

There are a number of substances derived from the malt or produced during the mash which are present in small quantities in our sweet wort. If they are allowed to remain, these substances can cause off-tastes in our finished beer. Fortunately the most objectionable of these substances are volatile and are driven off during the boll. However, it is important that an extremely vigorous boil is developed, with plenty of mechanical action, in order to ensure complete volatilisation of these unwanted products and to ensure that certain important oxidation processes take place A mere simmer is insufficient. The vigour of the boil is more important than the length of boil.

Coagulation of protein

One of the most important aspects of the boil is the coagulation and eventual precipitation of the proteins and other nitrogenous compounds in the wort. An excess of protein in the wort will give rise to hazes developing in the finished beer during storage. Protein hazes do not usually effect the flavour of the beer to any great extent, but they spoil the aesthetic appearance some-what, usually resulting in the pint being rejected by the drinker.

All real beer will contain some residual protein because the only effective way of removing it completely is by chilling the beer to force a protein haze to be thrown and then filtering it out It is quite normal for a haze to be thrown when beer is chilled excessively, but it is nothing other than a protein haze and it will usually disappear again when the beer warms up to its proper temperature. However, the tendency for beer to haze at room temperature increases with its age, the temperature at which it is stored, and the quantity of these nitrogenous substances remaining in the beer.

It is therefore desirable to ensure that the amount of protein and other nitrogenous compounds remaining in the beer is at a minimum. The boil has the effect of causing the protein to coagulate and form into globules or clumps, which will settle out when the wort is cooled. The longer the boil, the more protein will be precipitated, and the longer will be the shelf life of the finished beer.

Some of the components that contribute to head retention are high-order proteins so it follows that a boil for an excessively long period may produce poor head retention. Whether the head is considered important or not is mostly a regional preference.

Hot break

After the wort has been boiling for some time it begins to haze as the proteins come out of solution. If the heat was turned off again the majority of these proteins would go back into solution or remain in suspension. However, after a prolonged boil of an hour or more the molecular structure of the protein begins to break up and coagualte into a dark sticky substance. The colloidial nature of these small particles causes them to stick to each other and from this point on they rapidly grow in size until they are too large to remain in solution. Thus, when the heat is turned off they settle on to the bottom of the boiler. The point at which this all begins to happen is termed the hot break, and the protein matter which settles out is termed trub in brewerspeak.

Cold break

After the hot break has been achieved and the trub allowed to settle out the wort will "drop bright" and become clear. However, as the wort is cooled it will go cloudy again and more protein matter will precipitate out of solution as trub. The faster the wort is cooled, the greater is the quantity of this protein matter which drops out of solution. This is termed the cold break in brewerspeak. Again the trub is allowed to settle out before the wort is transferred to the fermentation vessel.

Crash cooling

Force-cooling of the wort after boiling has several benefits. The most important of these is that the wort is exposed to airborne contamination for a shorter period of time and is cooled below the optimum temperature for bacterial growth fairly quickly. Another benefit is that rapid cooling forces more potential haze forming protein to be precipitated out of solution.

To force more trub to be precipitated out of solution we must not only cool quickly, say within an hour, but we must also cool to quite a low temperature. The lower the temperature, the more trub will be precipitated. An important point to bear in mind is that fast cooling the wort by the means of topping up with water will not cause trub to precipitate; in fact the opposite is true, the increased volume will encourage the protein to remain in solution.

In the home brewing environment we are usually unable to cool our wort fast enough to cause a significant amount of trub to drop out of solution. Even if we could rapidly cool our wort it is unlikely to cause a significant increase in the quality of our beer, particularly if we employed copper finings during the boil. In general, we do not require the long shelf life, or the same degree of haze stability that commercial breweries require.

The main advantage, to us at least, of force cooling the wort is to reduce the time it takes to complete our brew. It can take several hours for a wort to cool to pitching temperature by natural means, and during that time the wort is dangerously vulnerable to bacterial infection.

The most common method of wort cooling employed by the home brewer is to transfer the wort from the copper into a fermenting bin and then to stand it in a bath of cold water. However, carrying hot liquids in a "squidgy" plastic container is a hazardous business, particularly if one is lugging it upstairs to the bathroom. Another method is to submerge a coil of thin-walled copper or plastic tubing into the hot wort and to run cold tap water through the coil.

Practical boiling 1 (whole hops)

In the home brew situation the 'copper' or boiler can often serve as the collection vessel to collect the sweet wort during mash tun run-off and sparging. Ideally, the sweet wort should not be allowed to cool before the boiling operation begins. If the copper is being used as the collection vessel, then as soon as sufficient wort has been collected to cover the heating element, the boiler should be turned on and set to maintain the temperature of the wort at about 77C. However, as soon as all the wort has been collected, the thermostat is set to maximum and the boil started.

As soon as the wort comes to the boil ("copper up" in brewerspeak) the first batch of hops should be added. Adding the hops will cause copious foaming and should be performed gradually. Excess foaming can be beaten back into solution. The next stage is to add any sugars that are called for in the recipe. The time at which the sugars are added is not important, but they should receive at least half an hour's boil. Half way through the boil is about right. By this time the fight with the copious foam generated when the hops are first added has passed, and there is still a significant part of the boil remaining.

The sugars should be added gradually to ensure that they go into solution properly without settling on to the element and possibly burning. Commercial invert sugar, which often comes in blocks, or glucose chips, should be broken up into the smallest practical particle size in order to ensure rapid solubility. This can be performed by means of a hammer or rolling pin as appropriate.

Boiling period

The wort should be boiled vigorously for a minimum period of one hour, preferably two, or until hot break occurs. Weaker beers generally require a longer boil than strong beers, sometimes as long as two and a half hours. A vigorous or "good rolling boil" is essential to allow volatile products to escape, and for the oxidation processes to occur. The violence of the boil is far more important than the boiling period. Prolonged boiling will drive off subtle flavour compounds, but under-boiling will produce an unstable beer; it is all a matter of compromise. It is generally accepted that optimum boiling conditions are achieved when the wort evaporation loss is about 10 per cent per hour, and about one-and-a-half hours' boil is considered to be about right under these conditions.

Do not put the lid on the boiler or you will end up covered in hops! The first half an hour of the boil must be attended and watched very carefully. Excessive foaming, after the addition of the hops, should be beaten back, or stirred in, with a metal frying pan spatula or similar implement. Resist the temptation to turn down the boil unless absolutely necessary to prevent spillage. Eventually the foaming will subside and the boil can resume at full throttle.

About fifteen minutes before the end of the boil the remainder of the hops and the Irish moss (it required) are added. The normal quantity of Irish moss added is 1 gram per gallon or 5 grams per 25 litre batch. Irish moss is supplied in two forms; either as flakes or a more finely ground powder. One dessert spoonful of flakes is about 5 grams, but the powder will occupy a smaller volume; about two teaspoons. It is probably best to weigh it if you have the facilities.

Late hops and soaking hops

Sometimes hops are added towards the end of the boil to restore aroma and flavour compounds that may be driven off. These hops are usually added to the boil during the last fifteen or twenty minutes. The recipe will usually specify the duration, if it does not, assume fifteen minutes.

A common technique used as a substitute for dry hopping is to add a quantity of hops at the very end of the boil, say during the last 30 seconds, and leaving them to soak in the hot wort for half an hour after the boil is completed.

Wort run-off

When the wort has been boiled for the desired length of time the boiler should be switched off and about half an hour allowed for the trub to coagulate or settle out. The trub can sometimes be seen as dark gummy particles of various sizes, either floating on the surface or dropping out of suspension. When the trub has coagulated, or settled out of suspension, the tap on the boiler should be gently opened and the wort allowed to flow into a receiving vessel situated at a lower level.

Care should be exercised to ensure that as much of the trub as possible is left behind in the boiler. The spent hops should settle on to the false bottom and act as a filter bed. The first runnings from the copper will often be turbid and should be returned to the copper until the wort runs bright. I do not recommend the sparging of hops. This runs the risk of extracting trub and other undesirable substances from them. If you wish to sparge yours, do it sparingly.

Practical boiling 2 (hop pellets)

The use of hop pellets present a problem. They disintegrate into a powder during the boil and therefore, unlike whole hops, cannot act as a filter bed to filter out the trub. However, hop pellets can be used in home brewing provided that care is taken to ensure that as much trub and hop debris as possible are excluded from the final beer.

The boil should proceed in the same manner as when using whole hops described above. After the boil, switch off the heat, cover the boiler with a lid of some sort and leave to stand for about half an hour or so to allow the trub and hop debris to settle onto the bottom. After the settling period, draw off the wort via the tap and strain it through a fine nylon sieve into the collection vessel. The sieve will trap any seeds or other debris floating on the suface or in suspension. It does not matter if a bit of debris gets carried across; the sieve will catch most of it, and that which does get into the wort probably will not matter much.

Wort cooling

Once all of your wort has been collected it is time for the cooling phase. As I mentioned earlier, forced cooling will cause a further precipitation of trub from the wort, but in the home-brewing environment we are generally unable to cool our wort fast enough to cause a significant amount of trub to drop out of solution.

Several methods of wort cooling have already been mentioned. If you are cooling the wort with the intention of forcing further precipitation of trub, it is important to perform the cooling operation with the wort in its undiluted state, that is, before any top-up water is added.

If you are cooling simply to quickly bring the wort to pitching temperature, it is still desirable to leave the wort in its undiluted state for the primary cooling stage; the higher temperature gradient will cause more efficient cooling. When the wort has cooled to between 40°C and 45°C, the top-up water can be added to bring the wort to the desired original gravity, and the resultant wort should then be pretty close to pitching temperature. Further precipitation of trub may have occurred during cooling. You may leave this in the fermentation vessel, or rack the wort off into another vessel, just as the fancy takes you. It probably does not make much difference unless you are under competition.

Wort aeration

It will be remembered from the discussion on yeast that the yeast requires dissolved oxygen in the wort for the first part of its life cycle. The action of boiling the wort drives off the dissolved oxygen present in the wort, thus some oxygen must be admitted back if healthy yeast growth is to occur.